Amplification of a signal having a high peak-to-average ratio (PAR) is generally considered to be inefficient. The reason is that a high power amplifier (HPA) is required for processing a significantly large amount of input back-off when amplifying such a signal. One solution to this problem has been to use a PAR reduction algorithm. The PAR reduction algorithm is commonly used in many radio frequency (RF) transmitters. The algorithm is also called a crest factor reduction (CFR).
The CFR is operated in several modes. Among them, a peak cancellation technique will be described. The peak cancellation technique searches a peak in a given signal to reduce or cancel the peak.
A vertical axis indicates a size (e.g., amplitude) and a horizontal axis indicates a time in FIGS. 1 and 2.
FIG. 1 illustrates an input signal 130 of a peak cancellation mode according to the related art.
The apparatus for canceling the peak searches a portion, i.e., a peak 140, having an amplitude that exceeds a preset signal threshold value 120 in the input signal 130. The technique generates a cancellation pulse using a parameter for a newly sensed peak, e.g., amplitude, phase, and position.
FIG. 2 illustrates a method for applying a cancellation pulse according to the above-cited technique.
The method subtracts the cancellation pulse 210 from the input signal 130 to reduce or cancel the peak 140. An output signal 220 having a reduced peak 140 is shown below with respect to FIG. 3.
FIG. 3 illustrates a noise filter signal 310 of a noise shaping filter according to the related art. A horizontal axis indicates a frequency and a vertical axis indicates a size (e.g., amplitude). The noise filter signal has two frequency allocations (FA) 330 and 340.
The noise filter considers an impulse, i.e., the noise filter signal 310 of the noise shaping filter as well as the parameter for the peak so as to calculate the cancellation pulse. Hereinafter, an impulse response of the noise shaping filter is named as a noise filter signal. The noise filter signal is used to cancel the noise of the input signal. In addition, the noise filter signal 310 is designed to match the input signal 130 with the power spectrum. According to such a method, the cancellation pulse does not generate much noise in a region except for a signal carrier bandwidth.
However, according to the CFR algorithm, the noise filter processes the peaks one by one. Therefore, the noise filter is configured to generate one cancellation pulse at the same time, and may process the peaks one at a time. The noise filter needs to have several configurations capable of generating the cancellation pulse in order to process the several peaks. However, when several peaks are generated within a short time according to characteristics of the signal, an appropriate process is required.